Glazing from compositions comprising transparent thermoplastic polymers, such as e.g. polycarbonate, offer many advantages over conventional glazing of glass for the vehicle sector and for buildings. These include e.g. increased fracture-proof properties or saving in weight, which in the case of automobile glazing makes possible a higher safety of passengers in the event of traffic accidents and a lower fuel consumption. Finally, transparent materials which contain transparent thermoplastic polymers allow a considerably greater freedom of design due to the simpler formability.
A disadvantage is, however, that the high transparency to heat (i.e. transparency to IR radiation) of transparent thermoplastic polymers in sunlight leads to an undesirable heating inside vehicles and buildings. The increased temperatures in the inside reduce the comfort for the passengers or occupants and can result in increased demands on the air-conditioning, which in turn increase energy consumption and in this way cancel out the positive effects again. In order nevertheless to take into account the requirement of a low energy consumption combined with a high passenger comfort, panes which are equipped with appropriate heat protection are necessary. This applies in particular to the automobile sector.
As has been known for a long time, the majority of solar energy falls to the range of the near infra-red (NIR) between 750 nm and 2500 nm, in addition to the visible range of light between 400 nm and 750 nm. Penetrating solar radiation e.g. is absorbed inside an automobile and emitted as long wavelength thermal radiation with a wavelength of from 5 μm to 15 μm. Since in this range conventional glazing materials—in particular thermoplastic polymers which are transparent in the visible range—are not transparent, the thermal radiation cannot radiate outwards. A greenhouse effect is obtained and the interior heats up. In order to keep this effect as low as possible, the transmission of the glazing in the NIR should therefore be minimized as far as possible. Conventional transparent thermoplastic polymers, such as e.g. polycarbonate, however, are transparent both in the visible range and in the NIR. Additives e.g. which have the lowest possible transparency in the NIR without adversely influencing the transparency in the visible range of the spectrum are therefore required.
Among the transparent thermoplastics, polymers based on polymethyl methacrylate (PMMA) and polycarbonate are particularly suitable for use as glazing material. Due to the high toughness, polycarbonate in particular has a very good profile of properties for such intended uses.
In order to impart to these plastics heat-absorbing properties, corresponding infra-red absorbers are therefore employed as additives. In particular, IR absorber systems which have a broad absorption spectrum in the NIR range (near infra-red, 750 nm-2500 nm) with a simultaneously low absorption in the visible range (low inherent colour) are of interest for this. The corresponding polymer compositions should moreover have a high heat stability and an excellent light stability.
A large number of IR absorbers based on organic or inorganic materials which can be employed in transparent thermoplastics are known. A selection of such materials is described e.g. in J. Fabian, H. Nakazumi, H. Matsuoka, Chem. Rev. 92, 1197 (1992), in U.S. Pat. No. 5,712,332 or JP-A 06240146.
Nevertheless, IR-absorbing additives based on organic materials often have the disadvantage that they have a low stability towards exposure to heat or irradiation. Thus, many of these additives are not sufficiently stable to heat to be able to be incorporated into transparent thermoplastics, since temperatures up to 350° C. are required during their processing. Furthermore, the glazing is often exposed to temperatures of more than 50° C. over relatively long periods of time during use, due to the solar irradiation, which can lead to decomposition or to degradation of the organic absorbents. Furthermore, the organic IR absorbers often do not have a sufficiently broad absorption band in the NIR region, so that their use as IR absorbers in glazing materials is inefficient, an intense inherent colour of these systems often additionally also occurring, which as a rule is undesirable.
IR-absorbing additives based on inorganic materials are often significantly more stable compared with organic additives. The use of these systems is also often more economical, since in most cases they have a significantly more favourable price/performance ratio. Thus, materials based on finely divided borides, such as e.g. lanthanum hexaboride, have proved to be efficient IR absorbers, since they have a broad absorption band combined with a high heat stability. Such borides based on La, Ce, Pr, Nd, Tb, Dy, Ho, Y, Sm, Eu, ER, Tm, Yb, Lu, Sr, Ti, Zr, Hf, V, Ta, Cr, Mo, W and Ca are described e.g. in DE 10392543 or EP 1 559 743.
However, their significant inherent colour is a disadvantage of these additives. After incorporation, the boride-containing additives impart to the transparent plastic a characteristic green coloration, which is often undesirable since it severely limits the margin for imparting a neutral colour.
To compensate the inherent colour, relatively large amounts of further colouring agents are often employed, but this impairs the optical properties of the composition and leads to a significantly reduced transmission in the visible range. This is undesirable especially in vehicle glazing, or is inadmissible in specific cases where the vision of the driver must not be impaired.
IR absorbing additives from the group of tungsten compounds which have a lower inherent absorption in the visible spectral range compared with the inorganic boride-based IR absorbers known from the prior art are furthermore known.
The preparation and the use of these substances in thermoplastic materials are described, for example, in H. Takeda, K. Adachi, J. Am. Ceram. Soc. 90, 4059-4061, (2007), WO 2005037932, JP 2006219662, JP 2008024902, JP 2008150548, WO 2009/059901 and JP 2008214596. However, the lack of long-term stability to exposure to heat has proved to be a disadvantage. While the instability of tungsten oxides to heat is known per se and has been described, for example, in Romanyuk et al.; J. Phys. Chem. C 2008, 112, 11090-11092, it has been found that when these compounds are incorporated into a polymer matrix, the absorption in the IR range also decreases significantly during storage of the corresponding polymer compositions, such as e.g. in a polycarbonate composition, in heat at elevated temperature.
For use of the compositions in the glazing sector, in particular for automobile glazing, however, it is absolutely essential that the corresponding IR-absorbing polymer compositions have a long-term stability to higher temperatures. Higher temperatures mean e.g. temperatures which an article of polycarbonate can assume under intensive solar irradiation (e.g. 50° C.-110° C.). It must furthermore be ensured that the composition can be processed under conventional process conditions, without the IR-absorbing properties already being reduced as a result.
It was moreover known to use heat stabilizers, such as, for example, phosphites, hindered phenols, aromatic, aliphatic or aliphatic-aromatic phosphines, lactones, thioethers and hindered amines (HALS, hindered amine light stabilizers) in thermoplastic materials to improve the processing properties.
WO-A 01/18101 discloses moulding compositions comprising a thermoplastic and a phthalo- or naphthalocyanine dyestuff which can contain antioxidants, such as phosphite, hindered phenols, aromatic, aliphatic or mixed phosphines, lactones, thioethers and hindered amines to improve the processing stability. In contrast to this, the present invention relates to compositions comprising inorganic IR absorbers based on tungsten.
EP 1266931 discloses organic IR absorbers in polycarbonate compositions in combination with phosphines. However, an indication of the combination of inorganic IR absorbers, in particular inorganic IR absorbers based on tungsten, with phosphines for stabilizing the absorbers in a thermoplastic matrix is not described in EP 1266931.
EP 1559743 describes polycarbonate compositions comprising inorganic IR absorbers based on borides in combination with heat stabilizers, such as phosphonites and phosphines, these additives serving to stabilize the polycarbonate matrix. Tungsten-based compositions are not described. It is not known that the abovementioned stabilizers have an influence on inorganic IR absorbers.
US 2006/0251996 discloses multi-layered sheets comprising a core layer comprising a thermoplastic polymer and an IR-absorbing additive, the IR-absorbing additive being a metal oxide. The core layer moreover can additionally contain heat stabilizers. A polymer composition with a zinc-doped IR absorber based on tungstate or with a phosphine-stabilized IR absorber according to a particular embodiment of the present invention and masterbatches stabilized with phosphines, however, are not described in US 2006/0251996. In particular, US 2006/0251996 also does not describe the use of a nanoscale IR absorber embedded in a dispersing agent.
In all the thermoplastic compositions with IR absorbers published to date, the heat stabilizer serves exclusively, however, to stabilize the particular polymer matrix—in particular during processing. By using these systems, the yellow coloration of the polycarbonate after exposure to light, as described in EP 1266931, can thus be limited.